Procedures for preparing photographic images in silver by diffusion transfer principles are well known in the art. In a typical procedure of this nature, an exposed light-sensitive silver halide emulsion containing a developable image is developed with an aqueous alkaline processing composition including a silver halide developing agent and a silver halide solvent to reduce exposed silver halide to silver while forming from unexposed silver halide an imagewise distribution of soluble silver complex which is then transferred at least in part, by imbibition, to a superposed image-receiving layer where it is reduced to image silver to impart thereto a positive silver transfer image. Procedures of this description are disclosed, for example, in U.S. Pat. No. 2,543,181 issued to Edwin H. Land. The image-receiving layer or silver-receptive stratum, as it is often called, may typically provide one of the vigorous silver precipitating environments such as described in U.S. Pat. Nos. 2,698,237 and 2,698,245 issued to Edwin H. Land. The silver-receptive stratum may be contained on a suitable support which may be opaque or transparent. Included on this support comprising the image receiving element may be one or more layers containing ingredients performing specifically desired functions, e.g., waterproofing materials, optical brighteners for enhancing the whiteness of highlight areas of the print, etc.
In typical procedures for preparing silver transfer images in the foregoing manner, the inner surface of the image-receiving layer is tightly adhered to a layer of hydrophobic material which may comprise the support itself or may be positioned between the support and the image receiving layer. The layer for hydrophobic material upon which the image-receiving layer is supported is at least difficultly permeable to water and vapor and is capable of restricting penetration of photographic processing compositions therein, at least for the time that these processing compositions are in contact with the print. In either instance, the image-receiving layer is tightly adhered or bonded to the hydrophobic material to provide a strong laminate which is not easily severable. Diffusion transfer processes employing an image-receiving element including an image-receiving layer supported on a hydrophobic layer, as described above, are disclosed, for example, in U.S. Pats. Nos. 2,719,791, 2,774,667, 2,789,054 and 3,220,835, all issued to Edwin H. Land.
Image-receiving elements prepared by such processes where the silver transfer image is concentrated primarly at the surface of the sheet may be utilized as printing masters. As disclosed in U.S. Pat. No. 3,220,837 issued to Edwin H. Land and Meroe M. Morse, since the silver transfer image is hydrophobic and oleophilic while the underlying sheet is hydrophilic and oleophobic, a greasy ink will adhere only to the silvered areas of the image receiving element. The differentially inked sheet can then serve as a printing master without further treatment.
In carrying out silver transfer processes of the foregoing description, it is extremely desirable that the silver be deposited in the silver-receptive stratum in such a manner that it is retained securely following image formation. Where the silver is deposited primarily at or near the surface, the resulting silver transfer print has been found to be susceptible to loss of density due to rub-off, e.g., upon applying a print coating solution such as is disclosed in certain of the above-mentioned patents and which is employed to wash and protect the transfer image. This unwanted removal of image silver and consequent reduction in density had previously often been observed by me to be more pronounced when the image was formed primarily at or near the surface and in the presence of a wetting agent and/or a print coating solution containing a wetting agent. This removal of silver was most pronounced in areas of dense silver deposit. Since the presence of a wetting agent in some way reduced the adhesion of the transferred silver to the image-receiving element, it was apparent to me that the use of a wetting agent, or at least many kinds of wetting agents, should be avoided in such surface areas where it is desired to form a silver transfer image retained upon an image-receiving element not subject to having a portion removed by subsequent handling.
Although not useful in the aforementioned method of preparing printing masters by silver diffusion transfer processing, I had also previously found that the problem of "rub-off" rub-off can be eliminated or appreciably reduced if the image-receiving element is so formulated that the silver is deposited deeper into the receiving layer. The desired deeper deposit of the silver may be obtained by providing a silver-receptive stratum comprising a plurality of layer containing progressively increasing concentrations of silver-precipitating nuclei as the distance from the surface increases, the greatest concentration of silver precipitating nuclei being in the layer closest to the support or underlayer containing the silver-receptive stratum. Image-receiving elements of this description are disclosed, for example in U.S. Pat. No. 3,567,442 issued to Edwin H. Land.
The present invention is predicated upon the discovery that the tendency of a wetting agent to weaken the adhesion or adherence of transferred silver which had in the past been considered undesirable may unexpectedly be used to advantage in photo-mechanical reproduction systems where printing masters are prepared by silver diffusion transfer processing, especially those where silver is primarily deposited within, rather than on, the image-receiving element.